1. Field of the Invention
This invention relates to a process for producing methylal which is utilized as a plasticizer for a resin, an analgesic, a perfume or the like. More particularly, this invention relates to a process for producing methylal with good efficiency from methanol and formaldehyde using a solid acid as a catalyst.
2. Description of Related Art
Methylal is synthesized by condensation reaction of methanol with formaldehyde or paraformaldehyde in the presence of an acidic catalyst. As a method for the synthesis of methylal, it is generally known to use an acidic catalyst such as hydrochloric acid, ferric chloride or zinc chloride. Moreover, in JP-B-40-15,005, a method is proposed by which the starting mixture of methanol with aqueous formalin solution is passed through a distillation column packed with a cation exchanger to enhance the reaction yield of methylal. Furthermore, in each of the specifications of U.S. Pat. Nos. 5,108,550, 5,291,989 and 5,498,318, a reaction system in which a catalyst-packed type distillation column is used is disclosed as a method of enhancing the reaction yield though it is not limited to the synthesis of methylal. Further, in U.S. Pat. No. 4,967,014, there is disclosed a method for efficiently synthesizing methylal by a reaction-distillation column system in which a liquid consisting of methanol, formaldehyde and water is forcibly circulated to a solid acid catalyst-containing reactor connected to a distillation column.
However, according to the method which has heretofore been proposed, such problems are caused in some cases that when methylal is synthesized from methanol and formaldehyde, the reaction yield is restricted by water produced as a by-product and that the production has to be discontinued when the catalyst ability is lowered. The productivity can be greatly restricted because of the treatment of the waste water produced as a by-product, and in some cases, the industrial practice has been accompanied by much complexity.
For example, the reaction where methylal is synthesized from methanol and formaldehyde using an acidic catalyst such as hydrochloric acid is an equilibrium reaction, so that in said reaction, the enhancement of reaction yield has been restricted by the presence of water produced as a by-product as shown by the following reaction formula:
2CH3OH+CH2O ⇄CH3OCH2OCH3+H2O.
Moreover, as a method of enhancing the reaction yield, JP-B-40-15,005 proposes that the starting mixture of methanol with aqueous formalin solution is previously subjected to acetalization using a cation exchanger and a part of the liquid flowing out of the reaction chamber in which the cation exchanger is used is fed again from a position above the place where the starting mixture is fed. However, when an ion exchange resin has been used over a long period of time, such problems have been caused in some cases that the particles of the resin are collapsed into fine powder and the catalyst ability is lowered by metal ion-substitution or the like. In the method of said Japanese publication, such a system is inconvenient since the distillation column is packed with a cation exchanger, and the production of methylal must be discontinued every time the catalyst ability is lowered.
Moreover, in U.S. Pat. No. 4,967,014, there is proposed such a method that methylal is synthesized by a reaction-distillation column system in which a liquid consisting of methanol, formaldehyde and water is forcibly circulated to a solid acid catalyst-containing reactor connected to the distillation column. In the method of said patent publication, the reaction yield is enhanced and, in addition thereto, since the reactor uses an external, forcible circulation system, the installation of a preliminary reactor enables the regeneration, exchange and withdrawal of the catalyst to be conducted in the preliminary reactor, whereby a long term continuous operation is made possible without discontinuing the methylal production. However, while the method of said patent publication is excellent for the efficient production of methylal, when treatment of the by-product water is taken into consideration, the concentrations of formaldehyde and methanol in the distillation column bottom product become high and a high-COD load waste water is generated. At the same time, in the continuous operation of the distillation column, a pressure rise due to bubbling in the distillation column is caused, and hence, an inconvenience has been caused in carrying out the industrial production of methylal.
The present inventors have extensively examined a method for the industrial production of methylal to find the usefulness of a solid acid catalyst and simultaneously have found the optimum conditions for the distillation column.
According to this invention, there is provided a process for producing methylal by contacting a liquid containing methanol, formaldehyde and water with a solid acid catalyst to obtain a component rich in methylal as a distillate, which process comprises feeding a liquid containing methanol, formaldehyde and water to the upper-most reactor of at least four solid acid catalyst-filled reactors which are each externally connected near to the intermediate portion of a distillation column; forcibly circulating a liquid containing methanol, formaldehyde, water and produced methylal in the temperature range of 80xc2x0 C. to 100xc2x0 C. between the distillation column and each of the other reactors which are connected to the lower side of the distillation column, whereby the vapor in the distillation column, which vapor has been subjected to vapor-liquid contact with the liquid containing methanol, formaldehyde, water and produced methylal and which liquid has been subjected to solid-liquid contact in any reactor connected to the lower portion of the distillation column, is successively subjected to vapor-liquid contact with the liquids containing methanol, formaldehyde, water and produced methylal circulated between the distillation column and the remaining reactors which are connected to the distillation column above the reactor in which the above vapor is generated, by which the methylal concentration in the vapor phase is successively increased; simultaneously therewith adding a defoaming agent from the top portion of the distillation column; adjusting the temperature of the bottom product of the distillation column to 100xc2x0 C. to 130xc2x0 C.; and withdrawing water produced as a by-product as waste water.
According to the production process of this invention, the synthesis of methylal in the prior art in which the reaction yield has been restricted by water produced as a by-product can be carried out in high yields. The process of this invention facilitates the treatment of water produced as a by-product, further enables continuous production to be achieved by addition of a defoaming agent, and hence, is very high in industrial value.